COCHLEAR IMPLANT – A BOON

M.S.R.M., a  39 year old engineer while supervising a construction work fell from a height of 20 feet.  Recovering from the injury, he experience diminished  hearing along with giddiness.  The deafness progressed till S.M. could not hear anything.  Hearing tests revealed bilateral profound sensory neural deafness not amenable to medical or surgical intervention.  Hearing aids did not benefit him.  He became the first recipient of a 22 channel cochlear implant in India which catapulted him from the world of silence to one of sounds.

Among the five senses, hearing is unique to humans, it enables  one to enables one to enjoy the melody of music, comprehend speech, abhor noise.  Deafness resulting from the defects confined to the outer and middle ear, termed conductive deafness is curable by drugs or surgery.  Deafness due to a lesion in the inner ear leads to sensory neural impairment which is nonreversible.  The patient, depending on the severity of the hearing loss, may be helped by amplification through a hearing aid.  A profoundly deaf person may not receive any benefit from a high power hearing aid rendering him totally handicapped, auditorily.

Cochlear implant has emerged as an effective option to the profoundly deaf.  The development of cochlear implant is attributed to the demonstration of   hearing  by electrical stimulation of the nerve.  The first cochlear implant was performed in 1978 at the University of Melbourne on a postlingually deaf adult (one who acquired deafness after developing speech). The patient’s ability to hear and comprehend speech heralded a milestone in the annals of ear surgery for sensory neural deafness. 

What is a Cochlear Implant?

A cochlear implant (Bionic Ear) is an artificial hearing device, designed to produce useful hearing sensations by electrically stimulating nerves inside the inner ear.

The multi-channel cochlear implants designed by the University of Melbourne and Cochlear Limited, consist of 2 main components: 1) the cochlear implant package and electrode array (or receiver-stimulator) and 2) the speech processor and headset. The model shown in the Figures below is the Nucleus® 24 cochlear implant system.

The implant package (or receiver-stimulator) contains the electronic circuits that control the flow of electrical pulses into the ear. It also contains an antenna that receives the radio-frequency signal from the external coil and a magnet that holds the external coil in place. Attached to the package are wire leads that join to electrodes. The 22-electrode array is inserted into the shell-like structure in the inner ear known as the cochlea. The ball electrode is placed under a muscle near the ear. There is also a plate electrode on the outside of the receiver-stimulator package. All of the parts shown in Figure 1. are placed under the patient's skin behind the ear during the implant operation.

Figure 1. The internal component of the cochlear implant

The other parts of the implant system are worn externally. The coil is held in position against the skin by a magnet and the microphone is worn behind the ear. The body-worn speech processor (SPrint^TM) can be worn in a pocket, in a belt pouch or in a harness. The ear-level processor (ESPrit^TM) is worn behind the ear. These parts, which are shown in Figure 2. are normally fitted two weeks after the operation.

Figure 2a. SPrint^TM speech processor

Figure 2b. ESPrit^TM speech processor

There are currently two cochlear implant systems made by the Australian company Cochlear Limited. The Nucleus® 22 system uses the CI22M implant and the Spectra 22 speech processor. This system has been available world-wide since 1984. Early in 1997, the Nucleus® 24 was released. This system (shown above) offers the choice of a body level (the SPrint^TM) and an ear level (the ESPrit^TM) speech processor with its CI24M implant. The Nucleus 24 system utilizes up-dated technology in both the implant and the speech processors.

How Does a Cochlear Implant Work

Speech and other sounds are picked up by the microphone and sent to the speech processor. The processor codes the sounds into an electrical signal which is sent via a cable to the transmitting coil. The coil passes the signal through the skin to the implant which transforms the signal to electrical pulses. The pulses pass from the electrode array and stimulate hearing nerve fibres within the cochlea.

The speech processor does not just make sounds louder as does a hearing aid. Instead, it selects out some of the important information in the speech signal and then produces a pattern of electrical pulses in the patient's ear. This pattern is selected to sound as close as possible to the original speech sound. It is not possible to make sounds completely natural, because there are only 22 electrodes that are replacing the function of tens of thousands of hair cells in a normally hearing ear.

The electrical patterns are different for each person and need to be programmed into the speech processor by a trained clinician. The differences arise because the electrodes are not always in the same position relative to the surviving nerves and the nerves vary in sensitivity to electrical currents. The clinician must measure the lowest and greatest current for every electrode to determine the softest and loudest sounds that will be heard. The different electrodes produce sounds with different pitch. The speech processor combines sounds on different electrodes with different loudness, to build up something as close to the original sound as possible.

The hearing process using a cochlear implant can be summarized as follows (please refer to Figure 3):

Figure 3. The hearing process using a cochlear implant

1. Sounds and speech are detected by the microphone.
2. The information from the microphone is sent to the speech processor.
3. The speech processor analyses the information and converts it into an electrical code.
4. The coded signal travels via a cable to the transmitting coil in the headset. Radio waves from the transmitter coil carry the coded signal through the skin to the implant inside.
5. The implant package decodes the signal. The signal contains information that determines how much electrical current will be sent to the different electrodes.
6. The appropriate amount of electrical current passes down the appropriate lead wires to the chosen electrodes.
7. The position of the stimulating electrodes within the cochlea will determine the frequency or pitch of the sounds. The amount of electrical current will determine the loudness of the sounds.
8. Once the nerve endings in the cochlea are stimulated, the message is sent up to the brain along the hearing nerve. The brain can then try to interpret the stimulation as a meaningful sound.

Who would benefit from cochlear implant ?

One with a profound sensory neural deafness as determined by audiological evaluations.

One who does not benefit from the most powerful hearing aid subsequent to optimisation of hearing aid trial and fitting.

One with no medical contraindication for surgical implantation of the electrodes.

If, in children with access to facilities for extensive speech and language therapy to develop auditory skills and  speech development.

One with a high degree of motivation and appropriate, realistic expectations.

The major factors contributing to the success of a cochlear implant are:

The length of profound deafness prior to the implantation.  This probably is related to the memory for sound.

The development of speech and language skills before acquiring deafness. Clinically, it has been demonstrated that people who have normal speech and acquired deafness subsequently (post-lingual deaf) do better with cochlear implants than those who are born deaf with non-development of speech (post-lingual deaf).   However, studies on pre-lingual deaf implantees have shown satisfactory results,  if the implantation is done at an earlier age (critical to the development of speech and language skills) and supplemented with adequate speech therapy.

The number of functioning nerves in the inner ear.

The speech and audiological  rehabilitation support from the clinic and the support from the family.

The motivation from the patient and his family and their commitment towards the rehabilitation programme.

 Contrary to general behalf, the cochlear implant does not restore  the recipient’s hearing to normal levels.  Further, the sound perceived through the implant will be different from that perceived by the normal cochlea.  The sounds have been described as being electronic, similar to a computer generated speech.  Hence, the need for the rehabilitation programme focussing on developing the auditory skills in the new mode.  It needs to be emphasised that communication with a cochlear implant is maximised in conjunction with lip reading rather than audition alone.

Patient selection

The selection of a person for a cochlear implant follows a rigorous protocol specified by the Cochlear Party Inc.  This is to ensure maximum benefit from the surgery and the device.  The patient needs to undergo a thorough E.N.T. evaluation to rule out any contraindications for the surgery.  A detailed audiological evaluation should be done to confirm the profound degree of sensory neural deafness in both the ears.  This should be  followed by a hearing aid trial and optimisation of hearing aid fitting.

An aided audiogram and speech discrimination with the selected hearing aid would be determined and re-determined after at least six weeks of using the hearing aid.  This procedure is required even if the hearing aid does not offer any significant benefit to the patient.  This is to ward off the criticism that the patient has been fitted with the cochlear implant without determining the effectiveness  of a suitable hearing aid.  A patient receiving little or no benefit with an optimised hearing aid selection procedure would be considered an ideal candidate for the implant.  A CT scan would assist the surgeon overcome problems, if any, with electrode insertion into the cochlea.

It takes about three or five weeks for the incision made for the implantation to heal.  The patient is now ready for the “switch on” of the device by the audiologist.  “Switch on “ “involves fitting the external headset and programming the speech processor.  The speech processor has to be individually programmed because the current required for sensation of hearing varies for electrodes implanted and also from patient to patient.  The programming or “MAP” ping” as it is technically known is done by a specially trained audiologist.  The mapping is done by using a diagnostic programming system and necessary software installed in the clinic.

The audiologist during mapping, establishes the T-levels, C-levels for each electrode and balancing loudness of all the electrodes.  The T-Level is the level at which the patient just perceives the sound sensation.  The C-level is the maximum stimulation level not  causing discomfort.   Balancing loudness ensures a smooth sound perception.  The MAP  thus created is a complete set of instructions to the speech processor to encode the signals which are to be presented to the electrodes.  The MAP is written into the memory of the speech processor and becomes active when the speech processor is switches on.  The audiologist would create new MAPs subsequently, comparing them with the MAP in the speech processor to provide the one best suited to the patient.

Rehabilitation after implant

The need for rehabilitation after cochlear implant is to help the recipient adjust to new sounds heard through the implant.  Further, rehabilitation helps in learning communication and listening strategies specific to cochlear implantees, device usage and to develop appropriate expectations.  Rehabilitation involves intense auditory training starting from the simple task of detecting the environmental sounds to identification and comprehension of conversation.  Initially, training in comprehension is done with closed set material where the patient can choose from a set of possible responses.  Over time, the patient is exposed to open set material offering no clues or choice.  The implantee would be first trained in the audiovisual mode i.e., with the speech processor on and lip reading and only then through auditory mode.  This would facilitate comprehension of speech in the absence of any visual clues, as in darkness.

Over 10,000 people including children have benefited from cochlear implants worldwide.  Recipients of nucleus multichannel implants have reported that it enabled them to hear environmental sounds and conversation at comfortable loudness levels.  Further, the cochlear implant enhances lip-reading ability and helps in speech production in children.  The benefits of 22 channel cochlear implants  have been better communication, increase confidence, more social interaction, improved job performance and opportunities.  Like any sophisticated medical device, cochlear implants are expensive, costing a few lakhs.  With more awareness and patient’s venturing to receive its benefit, the cost is bound to come down to affordable levels.